The Atlantic cod, Gadus morhua, has had its genome sequenced for the first time, revealing that it has a unique immune system. This finding, published in Nature this week, challenges the current assumptions about the evolution of the vertebrate immune system, and may have implications for the farming of fish.
The Major Histocompatability gene, or (MHC) II, is a conserved feature of the immune system in all jawed vertebratraes, whose genomes have been sequenced so far, including humans. But this gene has been lost somewhere in the ancestry of the Atlantic cod.
The (MHC) II gene is part of the adaptive immune system, the immune system with a 'memory' for previously encountered pathogens. Professor Kjetill Jakobsen from the Centre of Ecological and Evolutionary Synthesis at the University of Oslo led the study. He explained that "(MHC) II is heavily associated with immunological memory" and that "vaccine strategies are dependent on the (MHC) II function".
There are cases of non-functional (MHC) II in humans, but such mutations usually lead to death at a young age. "You cannot lose this without building up an alternative way to deal with bacterial infections and other parasites" explains Professor Jakobsen.
Cod, however, exist in large numbers and are widely distributed, proving that they manage to have a functioning immune system without (MHC) II. This may be due to over expression of other immune system genes; "In addition to these genes that are lacking, there are some amplications of certain genes" says Professor Jakobsen. These amplications have been found in the innate immune system, the immune system that deals with intracellular pathogens.
Unlike mammals, very few fishes species have been sequenced. Cod belongs to a group of fish, the gadoid group, which have had never been looked at before. Initial studies of other fish within the gadoid group are already underway, "It seems they also have the same peculiar setting in their immune system" says Professr Jakobsen, who is now building on the theory that the immune system feature may be common to the whole group of fish. If correct, the alteration must have occurred millions of years ago in some common ancestor.
Not only will this finding be of academic interest to evolutionary geneticists and immunologists, but may also help the farming of cod where bacterial infections have previously been problematic.
Click here to listen to Professor Kjetill Jakobsen discuss the work
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